1. Field of the Invention
The present invention relates to a method of voltage measurement and an apparatus for the same and especially to a method and apparatus of voltage measurement of each voltage supply of a plurality of voltage supplies connected in series.
2. Description of the Related Art
An electric motor vehicle has a high voltage supply which is formed with a plurality of batteries (voltage supplies) connected in series. JP,H11-248755,A discloses a flying capacitor method for measuring voltage of each voltage supply of a plurality of batteries.
Each voltage supply is connected to a capacitor with a switch and disconnected each other with a multiplexer to measure a charge in the capacitor and to determine the voltage of the voltage supply.
A measurement is achieved by charging the capacitor with every each voltage supply and measuring the charge of the capacitor as shown in FIG. 8. The measurement of the voltage supply V1 includes a period of time of charging to charge the capacitor with the voltage supply V1 and a period of time of charge measurement to measure the charge of the capacitor for measuring the voltage of the voltage supply V1. The following measurement such as V2 also includes the period of time of charging and the period of time of charge measurement.
The conventional apparatus includes a polarity offset circuit for the charged capacitor and A/D converter for the charged voltage of the capacitor. Another conventional apparatus includes a voltage divider circuit and a buffer filter connected to a capacitor and an A/D converter for removing noise.
However, in the conventional apparatuses, the devices following to the capacitor have variations in quality, temperature characteristic, and degradation thereof so that the measurement accuracy is reduced. For a high-accuracy voltage measurement, high accurate and high reliable devices are required and resulting that the apparatus becomes expensive.